Sporadopora dichotoma

Sporadopora dichotoma (Moseley, 1876)

(Fig. 12a; Fig. 17)

Polypora dichotoma Moseley, 1876: 94–95

Sporadopora dichotoma: Moseley 1879: 429–440, pl. 34, figs. 1–2, pl. 35, figs. 1, 2, 9, pl. 36, pl. 43, figs. 1–9, 12, pl. 44, figs. 13–14; Moseley 1881: 36–47, 83, pl. 1, figs. 1– 2, pl. 2, figs. 1, 2, 9, pl. 3, pl. 10, figs. 1–9, 12, pl. 11, figs. 13–14; Boschma 1957: 60–61; Boschma 1964: 61–62; Cairns 1983a: 65–67, figs. 1a–b, 2a–i, 3a–b, map 1; Cairns 1983b: 436–438, figs. 2a–h, 24f, 25h, 27a, 28c; Cairns 1991: 44–46; Cairns & Macintryre 1992: 98, table 1; Zamponi 2008: 188, 198, fig. 9; Río Iglesias et al. 2012: 191; Bax & Cairns 2014: 108–111, table 1, map 4

Distribution. Off Mar del Plata, Argentina, 1097 m; Patagonia, 42° S to 48° S; Scotia Arc from Tierra del Fuego to South Georgia, Malvinas Islands and Shetland Islands, 250–1498 m. New record off Mar del Plata, 854–2204 m.

Material examined. USNM 60098 off Burdwood Bank, Eltanin St. 1593 (54° 43–42’ S, 56° 37–39’ W); USNM 60100 off South Georgia, ARA Islas Orcadas St. 575–34 (54° 41.6’ S, 34° 51.1’ W); MACN-In 40650 off Mar del Plata, Argentina, St. 35 (37° 54.045’ S, 54° 24.091’ W), 1245 m, May 2013; MACN-In 40651 off Mar del Plata, Argentina, St. 59 (37° 49.688’ S, 54° 5.236’ W), 1398 m, September 2013; MACN-In 42502 off Mar del Plata, Argentina, St. 11 (37° 59.258’ S, 54° 41.436’ W), 854 m, August 2012; MACN-In 42503 off Mar del Plata, Argentina, St. 42 (37° 59.110’ S, 54° 41.136’ W), 877 m, May 2013; MACN-In 42504 off Mar del Plata, Argentina, St. 37 (37° 59.848’ S, 54° 24.206’ W), 1275 m, May 2013; MACN-In 42505 off Mar del Plata, Argentina, St. 41 (38° 01.631’ S, 54° 30.275’ W), 997 m, May 2013; MACN-In 42506 off Mar del Plata, Argentina, St. 25 (37° 51,688’ S, 54° 10,550’ W), 1950 m, August 2012; MACN-In 42507 off Mar del Plata, Argentina, St. 56 (37° 54.840’ S, 54° 2.470’ W), 2204 m, September 2013; MACN-In 42508 off Mar del Plata, Argentina, St. 60 (37° 51.700’ S, 54° 4.583’ W), 1584 m, September 2013.

Description. Colonies robust and large. Uniplanar growth, although some deviations from main plane may occur. Branching dichotomous with ocassional anastomosis. Initial growth form is a branchless column. First dichotomous division occurs at around 5 cm height. Branches round to slightly elliptical in cross section, with larger axis generally parallel to plane of growth. Largest basal fragment 3.2 x 3.0 cm wide in cross section, whereas thinnest branches may measure only 7 mm. Branches round-tipped so that at an incipient dichotomic division new ones resemble lobes (Fig. 12a). No lateral branchlets, only apical division, so decrease in diameter from main to secondary branches is gradual. Some colonies grow attached to small rocks through non-expansive base and new coenosteum may form over dead one.

Branches smooth and spineless, although some specimens may bear a more granulate apical zone. Coenosteum white and porous, with a rudimentary texture under SEM. Coenosteal strips 25–60 µm wide, lack granules or platelets, and form reticulate pattern.

Pores of two kinds: round ones and irregularly-shaped ones. Larger round pores are gastropores and smaller ones are dactylopores (Fig. 17e, c). Irregular pores are the openings of efferent ducts, variable in size (Fig. 17f, c). All types of pores uniformly distributed on surface of terminal branches, but in thicker branches pores concentrate on anterior face, and in basal branches a considerable section of basal zone may lack pores entirely. Gastropores 0.26–0.66 mm in diameter (average 0.49 mm, n=46, σ=0.09). Gastropore tubes cylindrical, long and axial and curve towards surface of coenosteum in upper section (Fig. 17b). Dactylopores 0.16–0.23 mm wide (average 0.19 mm, n=31, σ=0.01). Dactylopore tubes long and axial and curve towards surface like gastropore tubes. Dactylopores slightly raised from surface in apical zone of some specimens, resulting in a rough surface of coenosteum. No tabulae or dactylostyles.

Gastrostyles needle-shaped and almost as long as tube. Gastrostyle illustrated in Fig. 17d is 1.1 mm high and bears longitudinal ridges of short spines fused with each other.

Ampullae elliptical and completely sunken in coenosteum, with larger axis perpendicular to surface (Fig. 17a). They may measure up to 1.2 mm x 1.0 mm. Given the size of ampullae and of efferent pores, specimens are probably female.

Discussion. Genus Sporadopora was originally described by Moseley (1876) as Polypora based on specimens collected off Mar del Plata in the Challenger expedition, but Moseley (1879) replaced this name by Sporadopora, since Polypora was preoccupied for a genus of bryozoans. Cairns (1983a) provided the second record of this species based on specimens from Mavinas Islands, Scotia Arc from Tierra del Fuego to South Georgia and Shetland Islands. The next record is from Río Iglesias et al. (2012) based on specimens from Patagonia from 42° S to 48° S within a depth range of 200–1500 m (specific depth not informed). The present work provides the fourth record of S. dichotoma and extends its known bathymetric range of occurence to 2204 m depth.

Dactylopore size range of our specimens is higher than that reported by Cairns (1983a). However, Cairns (1983a) stated that the upper limit of dactylopore size range is unknown and that it may overlap with that of gastropores. Tabulae were not found along the examined gastropore tubes, contrary to what Cairns (1983a) described in some cases. It is probably necessary to examine more material and to use an alternative method of wearing down the gastropore tube wall, since sandpaper may have caused tabulae to dissapear in the examined fragments. Despite these differences, specimens coincide with S. dichotoma in growth form, coenosteal texture, pore types and shapes, gastropore and dactylopore tube shape and ampullae characteristics.